1. Field of the Invention
The subject invention relates to the manufacture of CMOS transistors, and more particularly to improvements in the field oxidation step, well drive, field stop drive-in and latch-up, and radiation tolerance. It is equally applicable to wafer production or the manufacture of one or a few individual VLSI devices.
2. Prior Art
For years the prior art has been concerned with the "birdsbeak" effect and the effect of parasitic transistors, but to date only minimal progress has been made in eliminating either of these undesirable effects when growing the field oxide.
In the fabrication of devices with localized oxide (LOCOS) isolation, a layer of oxide (SiO.sub.2) and a layer of nitride (Si.sub.3 N.sub.4) are commonly used as a selective oxidation barrier during field oxide growth. The areas not protected by such layers are exposed and oxidized.
Ideal masking by the oxide and nitride against the field oxidation does not occur due to the diffusion of oxygen and the growth of SiO.sub.2 (oxide) under the nitride layer. As a result, a ridge of SiO.sub.2 protruding above the wafer surface is formed around the perimeter of the nitride mask. The profile of the oxide ridge in cross section appears as a birdsbeak. The beak is undesirable, since it creates an abrupt surface feature which can generate breaks or discontinuities in subsequent layers. Furthermore, the birdsbeak generates tapered side walls and this in turn generates a parasitic edge transistor.
The shape of the birdsbeak and the abruptness of the side wall angle depend on the following factors: (1) the thickness of the oxide between the silicon nitride and the silicon surface; (2) the quality of the silicon nitride film; (3) the thickness of the silicon nitride film; (4) the amount of silicon etched from the substrate (field region) prior to field oxidation; and (5) the temperature at which the field oxide is grown.